1. Field of the Invention
The present invention relates to a trim and tilt device for an outboard motor, and more specifically, relates to a configuration for adjusting the hydraulic pressure in the trim and tilt device.
2. Description of the Related Art
An outboard motor in a boat and the like is mounted on a stern via a tilt and trim device capable of adjusting a mounting angle of the outboard motor to a hull. This tilt and trim device includes a device body including a molded portion, a tilt cylinder, a trim cylinder, and a hydraulic pressure pump which are mounted on the device body. A hydraulic route for allowing the outlet side of the hydraulic pressure pump to communicate with the tilt cylinder and the trim cylinder is provided in the device body.
The tilt cylinder is rotatably mounted via a clamp shaft to a clamp bracket fixed to the stern. For example, two trim cylinders are arranged to have the tilt cylinder therebetween. In a trim range, a piston rod is raised since a piston in the trim cylinder is pushed up, and the piston rod contacts a tilt bracket mounted on the clamp bracket. This allows the outboard motor to be raised in the trim range. The tilt bracket is supported by the clamp bracket to pivot about a horizontal shaft. The outboard motor is supported by the tilt bracket.
Generally, this kind of boat travels such that the outboard motor can be raised or lowered using the trim cylinder, that is, in a trim range. During acceleration of the boat, the outboard motor travels in a raised (pivoted) state within the trim range. When the boat travels in shallow water, the outboard motor is raised to a tilt range in order to prevent the propeller from contacting the bottom. After passing through shallow water, an operator lowers the outboard motor to the trim range to travel again under normal conditions.
This kind of tilt and trim device for an outboard motor is disclosed in JP-A-2002-308184. This tilt and trim device can be used for the tilt operation and the trim operation of the outboard motor, and the boat can travel under normal conditions.
However, after passing through shallow water, some operators fully open an accelerator before the outboard motor is lowered to the trim range. This causes the outboard motor to be lowered while a propeller rotates under maximum output power. When the propeller is lowered to the water surface in such a state, the propeller churns water suddenly, and thrust under maximum output power of an engine is generated. At the same time, a reaction force of the thrust acts on the outboard motor and attempts to rapidly lower the outboard motor.
At a position in the tilt range when the propeller is lowered to the water surface, a load of the outboard motor and the reaction force are received only by the tilt cylinder. When the outboard motor is rapidly lowered from that position, hydraulic pressure in the tilt cylinder flows into the trim cylinder and the reserve tank that communicates with the tilt cylinder. The hydraulic pressure at this time is high, so that it is necessary to suppress the inner pressure of the trim cylinder with a relatively low pressure resistance to not greater than a specified pressure.
Generally, in a state in which the piston rod of the trim cylinder contacts the tilt bracket, the trim cylinder can sufficiently withstand the high hydraulic pressure caused by the reaction force of the thrust of the propeller even when the accelerator is fully opened since the high hydraulic pressure is received by a piston supported by the tilt bracket via the piston rod. However, in a case that the trim cylinder receives the reaction force of the thrust of the propeller under a fully-opened state of the accelerator before the piston rod of the trim cylinder contacts the tilt bracket as described above, as a piston in the trim cylinder is unsupported, the pressure generated in the tilt cylinder is rapidly transmitted to the trim cylinder so that the rapid pressure increase occurs.
In order to avoid this, it is conceivable to increase the pressure resistance by thickening the trim cylinder. However, the width of the exiting tilt and trim device is predetermined. Consequently, the size of the trim cylinder is limited, and it is difficult to increase the pressure resistance by changing the size of the trim cylinder itself. Moreover, thickening of the trim cylinder is not preferable since it goes against the recent development of a lightweight tilt and trim device.